Add RTL specific styles

[yuriy-fix]

Fixes #854

[web-padawan]

Let's update one of the dropdown visual tests to have selected item, to test checkmark appearance.

I would rename "Plain template" visual test to "Selected item" and change the label accordingly, because actually that combo-box does not use any <template> at all.

[DiegoCardoso]

I found one weird behavior where, if you set to rtl and back, some of the elements where still keeping themselves on rtl mode.

Steps to reproduce:

1. Open demo (eg "Presentation")
2. Call document.documentElement.setAttribute('dir','rtl')
3. Open the dropdown to verify it works
4. Call document.documentElement.removeAttribute('dir')
5. Open the dropdown

It will look like this (you can see by the text-field, that it's on ltr mode):

If combo box is never opened, then switching dir back and forth won't cause the issue. It seems to happen only after the overlay has been initialized.

[yuriy-fix]

The problem was coming from DirMixin: vaadin/vaadin-element-mixin#34

[DiegoCardoso]

🥇